I bought a used conrad-johnson D/A-2b. I had told admin that I would review the Mullard 6GK5 tubes I had planned on installing.

Upon receipt of the unit, I decided to add it to the system as is: I was not disappointed. Listening to various well recorded CD's, I was uttering "wow". Unfortunately, it was after about 5 hours over 2 days when the original (like from c-j) GE 6GK5 tubes failed. I decided to replace the now-defective GE tubes with the Mullards I had purchased and had planned on using. I knew from previous experience with my PV-12A that new Mullard tubes would take time to break-in. Finally, the system was ready. Tubes and cables appeared to be broken in, dampers on every tube in every component.

The Mullard 6GK5 Tubes

So how do the Mullards sound? I started by playing well known and well recorded CD's. Detail was good. Low frequency extension was good. Mid and high frequencies were generally good, but a tad bright. Moving on to CD's that were recorded okay (but not great) many of the same attributes held, although not to the same extent. Poorly recorded CD's were still poorly recorded CD's, and the D/A-2b exposed that over and over again. Mirroring iTunes to an Airport Express with Toslink out was not significantly inferior to the CD.

As time went on I noticed a disturbing effect when I increased the sound level. I was not pleased with the sound of many (even good) recordings in mid frequencies. It seemed that all was fine until the sound level reached in excess of 96dba, at which point it sounded rather harsh. I recalled how the D/A-2b with the original GE tubes played very well at an increased volume on many of the same recordings. So something needed to be changed. I ruled out cables, microphonics, clipping (the PV-12A is typically at 8 o'clock at that level and the Premier 11A is loafing), tube decay (less than 500 hours on pre and power amp tubes), and anything else I could think of. I decided to switch out the Mullards, so I ordered and installed a pair of NOS GE's.

The NOS GE 6GK5 Tubes

After allowing the GE's to break-in, I played the same music at the higher sound levels that had been disappointing with the Mullards. Immediately, I noticed a huge difference in warmth.; the somewhat harsh mids are now pleasant. Low frequencies are very good, although not quite as tight as with the Mullards. Detail is simply exceptional, better than the Mullards. Background instruments came to life, so much so that my wife commented that there were instruments she had never heard before in songs we've played 100's of times.